A major concern with construction work zones such as shafts is the control of groundwater particularly at the junction of water-bearing soils and the underlying rock. Considerable efforts have been made to find means for satisfactorily reducing or minimizing soil erosion when excavation proceeds through such a junction.
Many geotechnical construction techniques exist for forming subterranean barriers that provide a groundwater cutoff and/or supporting excavation for a work zone. These techniques include installing sheet pile walls, cement/soil bentonite slurry trench cut off walls, concrete slurry walls, soil mixed walls, permeation grouted barriers, jet grouted barriers and artificial ground freezing walls (freeze walls).
At certain construction or excavation sites, digging must be conducted sufficiently deep where groundwater is encountered. When no barrier is provided to the groundwater flow, excavation below the groundwater table will result in ground subsidence and flooding of the excavation site. Artificial ground freezing is a technique where a perimetrical artificially frozen wall is created about the excavation site to act as a barrier against groundwater flow into the excavation site. With reference to FIGS. 1-2, an excavation site 10 is shown which extends through the waterbearing strata (zone) 12. The waterbearing strata are formed of soil and rock materials with varying permeability therein prior to freezing of the ground. To achieve artificial ground freezing, a plurality of freeze pipes 14 are disposed about the excavation site 10, prior to excavation. The freeze pipes 14, depending on the specific application, may be provided with sufficient length to extend to or into bedrock 16. Groundwater flow passes through soil material 18 which is located within the groundwater table 12 above the bedrock 16 which is assumed (generally) to provide an impervious barrier to groundwater flow. As such, there are minimal concerns with groundwater flow below frozen ground 26.
The freeze pipes 14 form a continuous circulating path for chilled brine which has a freezing temperature below that of water. A refrigeration unit 20 is provided on-site which allows for the chilling of the brine and pumping thereof through the freeze pipes 14. To artificially freeze the ground, particularly the soil material 18, the chilled brine is continuously passed through the freeze pipes 14. With reference to FIG. 2, with passage of an initial period of time, such as three weeks, some freezing 22 of the soil material 18 located about the freeze pipes 14 is achieved. With passage of additional time, such as six weeks, additional freezing occurs such that a continuous frozen barrier 24 may be defined about the freeze pipes 14. With further passage of time (e.g., 12 weeks), a solid robust barrier of frozen ground 26 may be created which has sufficient thickness and strength to provide support for the excavation in addition to preventing flow of groundwater therethrough. With a sufficiently robust barrier being formed, work may be performed at the excavation site 10 within the robust barrier 26 without ground subsidence or groundwater entering therein. The chilled brine is continuously pumped through the freeze pipes 14 to ensure that the robust barrier 26 is maintained during excavation.
It has been found that the soil material 18 may freeze at different rates. Several factors may contribute to this variation, including the rate of groundwater flow which may vary at different elevations in the soil material 18. In particular, a fairly rapid flow of groundwater may be difficult to freeze using artificial ground freezing techniques. To ensure that the soil material 18 is sufficiently frozen to permit excavation, temperature measurements are taken along the elevation of the soil material 18. For example, with reference to FIGS. 3A and 3B, isotherms “T” in the soil material 18 may be determined by measuring temperature at various locations about the freeze pipes 14. Where sufficient freezing has been achieved of the soil material 18 with negligible groundwater flow, the isotherms “T” define a fairly regular pattern as shown in FIG. 3A. However, with reference to FIG. 3B, where sufficient freezing has not been achieved, e.g., due to groundwater flow, the isotherms “T” about the freeze pipes 14 are distorted. As shown in FIG. 4, the isotherms “T” may be plotted vertically to evaluate the depth at which insufficient freezing may exist. Where a spot or area of insufficient freezing “A” is identified, an opening (so-called “window”) may be located through the frozen wall through which groundwater may pass.
Moving groundwater is generally recognized as the most adversarial condition for ground freezing, resulting in freeze formation (closure) difficulties and, if undetected, freeze failures. Movement of groundwater during freeze formation puts an extra heat load on the freeze pipes and the refrigeration plant, preventing or requiring more time to achieve “closure” and a continuous frozen wall. Where the groundwater velocity is high, groundwater flowing past a single freeze pipe transfers the cooling effect downstream which, in plan view, results in an egg-shaped formation of frozen soil around the pipe, growing more slowly on the upstream side (FIG. 3B). Where the groundwater velocity is sufficiently high, groundwater flowing past a single freeze pipe introduces such a large amount of heat energy that growth of a frozen zone into the stream of the groundwater flow is inhibited.
Generally, a pre-freezing groundwater velocity (Darcy velocity) greater than 1 to 2 m/day may result in a freeze with window(s) which must be closed by other means. For example, via additional refrigeration effort with additional freeze pipes that employ chilled brine or the use of an alternate freezing agent (e.g., liquid nitrogen), reducing groundwater gradients, or most commonly, grouting to reduce the ground permeability.
Where groundwater velocity in excess of 1 to 2 m/day is suspected, periodic temperature profiling of the freeze pipes is performed by measuring the static and stabilized brine temperature approximately every 0.5 m within each of the freeze pipes. Anomalous warm spots may be an indication of a window in the freeze wall. However, as the temperature profiling is obtained from within the freeze pipes themselves, at the time a potential closure problem becomes evidence, the frozen wall will typically be on the order of 2 to 3 m thick with access to the location of the window limited by the presence of frozen ground. Hence, additional boreholes are drilled at least 3 m away from the alignment of the freeze pipes. In some instances, warm spots are well defined and the location of the window can be precisely located, although oftentimes, the window is not well defined and requires application of additional measures such as grouting to a broad area with a “shotgun” type of approach. In short, closure of a window in the freeze wall requires additional measures that not only delay construction but raise the cost associated therewith substantially.
Conventional permeation grouting techniques and materials have several shortcomings. Namely, the setting of conventional grouts (both cementitious and chemical) is significantly delayed at colder temperatures. This renders conventional grouts less effective as they permeate the soil and approach the colder temperatures at and near the window. Also, permeation grouting cannot be performed with a grout which consists of one or more individual components that have a relatively low freezing point as the grout may not set at the temperatures encountered in the window. Additionally, conventional permeation grouts that are aqueous suspensions or solutions and water soluble (e.g., cement-based grouts, silicate grouts and acrylate grouts) are susceptible to dilution by the groundwater flow. Also, such conventional permeation grouts can be adversely affected by groundwater chemistry.
Cementitious grout (e.g., cement grout with bentonite and other additives) is commonly used as it is readily available, easily mixed with standard equipment and the unit cost of material is low. Cementitious grout is pumped into surrounding soil material to form an auxiliary barrier against the groundwater flow and cause the window(s) in the frozen wall to be sealed from the groundwater flow. However, cementitious grout generates a heat of hydration which can be confused with the warmth of flowing groundwater, making it difficult to evaluate the conditions as the work proceeds. Further, the heat of hydration is counterproductive to freezing in that it introduces additional heat to the situation. Under sufficiently cold situations, the cementitious grout may freeze or set (or at least have components thereof set). With the discharge of the cementitious grout into the voids of the surrounding soil materials, slow or no setting may result in a discontinuous grout matrix. Moreover, the groundwater flow may cause non-set grout to wash away. In fact, the quantity of grout required for some projects indicate that in some instances the grout is most likely effective by altering the permeability of the surrounding groundwater regime rather than by directly plugging the window(s).
Thus, there is a need for improved methods of inhibiting subterranean groundwater flow through a window or opening in frozen soil.